Conventional electrophotographic processes generally use a method in which an electrostatic latent image is formed on a photosensitive member or an electrostatic recording member by using various means, a toner is adhered to this electrostatic latent image, and then the electrostatic latent image is developed.
In this development, carrier particles called “carriers” are mixed with toner particles so as to charge each other triboelectrically, and thus positive or negative electric charge of an appropriate amount is imparted to the toner. Carriers are classified roughly into coated carriers that have a coating layer on their surface and uncoated carriers that do not have a coating layer on their surface. Coated carriers are superior to uncoated carriers in light of, for example, the lifetime of the developing agent.
Among the various properties required for a coated carrier that are particularly important are a property of imparting an appropriate charge (the amount and the distribution of electric charge) to a toner, a property of keeping the appropriate charge on the toner for a long period of time, and for this purpose, shock resistance, abrasion resistance, a property of sufficiently inhibiting formation of spent toner, and a property of preventing a change in the charge on the toner by resisting environmental changes such as humidity or temperature. Various coated carriers have been proposed.
For the purpose of providing a long-lived coated carrier, JP S61-80163A, for example, proposes a technique in which a surface of a carrier core material is coated with a resin such as a copolymer of a vinyl monomer and fluorinated alkyl (meth)acrylate containing nitrogen, or a copolymer of fluorinated alkyl (meth)acrylate and a vinyl monomer containing nitrogen. JP H2-24670A, for example, proposes use of a copolymer of a vinyl monomer and fluorinated alkyl (meth)acrylate containing nitrogen, and a copolymer of fluorinated alkyl (meth)acrylate and a vinyl monomer. Furthermore, JP H6-11908A, for example, proposes a solvent-soluble fluorine-containing polymer having an imide bond. According to these, a coated carrier with a relatively long lifetime is obtained by coating a surface of the carrier core material with a copolymer of a nitrogen-containing monomer and a fluorinated monomer, or a solvent-soluble fluorine-containing polymer having an imide bond. However, the coated carrier may not withstand use for a long period of time when these copolymer resins are used. This is because the adhesive strength of the coating resins at an adhesive interface with the carrier is poor due to an influence of a low surface energy of the fluorine group, and because sufficient shock resistance cannot be obtained due to the insufficient strength of the resins as a coating agent. Furthermore, in order to charge the toner negatively, the amount of added fluorinated monomer needs to be kept small, and thus it was not possible to obtain a charged member having a sufficiently low surface energy while imparting a sufficient charge to the toner. Consequently, over a long period of use, formation of spent toner of the toner or an external additive to the charged member cannot be inhibited sufficiently. Thus the charge characteristics deteriorate with the time of usage, so that problems such as image fog or density non-uniformities are caused. “Formation of spent toner” refers to the phenomenon that, for example, a toner, an external additive and/or a colorant are/is firmly adhered or fused on a surface of a charged member due to heat generated by mechanical collision or friction, for example, between particles or between a particle and a developing device.
Furthermore, JP H7-325426A, for example, proposes a long-lived coated carrier in which a fluororesin is mixed and used together with a resin conventionally used as a coating material of a carrier for electrophotography (for example, an acrylic resin, an epoxy resin, styrene, a styrene-acrylic resin, or a silicone resin) as a binder or a primer, so that the poor adhesive strength of the fluororesin as described above is reinforced.
However, when the fluororesin, which is more negative in the triboelectric series, and the binder resin, which is more positive in the triboelectric series, are mixed to coat the surface of the carrier core material as described above, there is a difference in the characteristics such as the melting point of these resins. Thus a uniform resin coating layer is difficult to obtain, there is a broad distribution of charge amounts, and image defects such as fog or toner scattering are caused, and the transfer efficiency may be lowered.
Furthermore, fluororesins have the characteristic of moving to the outermost layer of the resin coating layers when mixed and used with another resin, and thus the charge amount decreases significantly for a negatively charged toner. In addition, when the coating layer is stripped by a long period of use, the fluororesin is stripped first, and the binder resin appears at the outer layer with the time of usage, and thus a change in, for example, charge characteristics becomes significant.
A carrier coated with a silicone resin coating layer has been proposed conventionally as a carrier having a relatively low surface energy. Although formation of spent toner of a toner tends to occur less if the coating layer is made of a silicone resin due to its relatively low surface energy, the effect is not sufficient. Furthermore, due to its low surface energy and high insulation, the charge amount is extremely difficult to increase, and image defects such as fog or toner scattering tend to occur.
Use of a silane coupling agent has been proposed in order to improve the adhesiveness of a resin coating layer to address image deterioration (for example, insufficient image density or image fog defects) caused when the resin coating layer on a surface of a carrier is stripped or lost due to, for example, collision between carriers or friction between a development box and a carrier due to stirring (JP S60-19156A). Although the adhesiveness of the coating layer is improved, there is a problem that toner scattering or image fog is caused due to fluctuation in the charge amount under various environmental conditions.
For the purpose of improving the adhesiveness between a carrier core material and a silicone resin, JP S62-121463A, for example, illustrates a carrier in which a coating layer made of a silicone resin is provided on a surface of a carrier core material treated with a silane coupling agent. However, the outermost surface of this carrier is not provided with a component of an aminosilane coupling agent containing an effective amino group, and thus the carrier cannot impart charge to a negatively charged toner sufficiently, and scattering is caused at the time of printing. Thus, a satisfactory carrier still is not obtained.
Furthermore, Japanese Patent No. 2744790, for example, proposes a carrier that is coated with a silicone resin containing an aminosilane coupling agent, for the purpose of preventing a decrease in the charge amount on a toner in a highly humid atmosphere and of improving durability of a developing agent, when used in combination with a toner with its components limited. A decrease of the charge amount throughout its lifetime can be improved by the limitation of the toner components and the effect of the aminosilane coupling agent. However, the formation of spent toner of the toner is not inhibited sufficiently, although it tends to occur less.
Recently, JP H5-134467A, for example, has proposed a resin layer containing an aminosilane coupling agent that is double-coated and in which the components or additives in the resin of the intermediate and the outermost layer are different.
Furthermore, JP H5-204189A illustrates a carrier characterized in that a density gradient of, for example, a silane coupling agent is provided in a thickness direction of a silicone resin layer. The carrier does not have uniform constituents in the carrier resin layers, and thus the silicone resin-coated carrier particularly changes over time when left standing, and a difference in hardening appears between the outermost layer and the intermediate layer of the resin layers. Therefore, a significant difference in charge characteristics appears between toners from the initial stage of production and toners after a certain period of time, the charge amount decreases at high humidity when a conductive material is added, and carrier resistance changes significantly if the resin layers are stripped or lost at the time of printing. Thus, in the final evaluation, it cannot be said to have durability.
Furthermore, JP H7-104522A proposes a resin-coated carrier for a developing agent of electrophotography characterized in that a carrier core material has a resin coating layer made of a silicone resin or a modified silicone resin containing an aminosilane coupling agent, in that the aminosilane coupling agent is present in the coating resin in a range of 6 to 25 weight percents, and in that the equivalent weight of amino groups in the aminosilane coupling agent ranges from 163 to 235. In this technique, a base resin of the resin coating layer containing the aminosilane coupling agent is a silicone resin or a modified silicone resin. Examples of the modified silicone resin include various modified silicone resins such as an alkyd resin, a polyester resin, an epoxy resin, a polyurethane resin, and an acrylic resin. These base resins cannot inhibit sufficiently formation of spent toner of a toner or an external additive to charged members over a long period of use, and thus their charge characteristics deteriorate with the time of usage, so that problems such as image fog or density non-uniformities are caused.
For the purpose of obtaining a negatively charged carrier (a positively charged developing agent) whose triboelectrical charge characteristics are excellent and in which stripping tends not to occur, JP S60-213961A proposes a carrier in which a coating layer containing a terminal perfluoro alkylsilane coupling agent in a silicon varnish is formed on a core surface. However, the silicon varnish and the terminal perfluoro alkylsilane coupling agent are difficult to apply uniformly, and thus the coating layer tends to be nonuniform, such as generated when a fluororesin and a binder resin are mixed and used together as described above. Consequently, there is a broad distribution of charge amounts, and image defects such as fog or toner scattering are caused.
Japanese Patent No. 2801507 proposes a carrier in which for a positively charged toner, a fluorine-substituted alkyl group is introduced to a silicone resin of a coating layer. Furthermore, as a carrier in which development properties in a high speed process are high and whose properties are not deteriorated over a long period of time, JP 2002-23429A proposes a coated carrier containing conductive carbon and a crosslinked fluorine modified silicone resin. This carrier takes advantage of excellent charge characteristics of the silicone resin, imparts characteristics such as sliding properties, stripping properties due to the fluorine-substituted alkyl group, and water-repelling properties, tends not to cause abrasion, stripping, or cracks, and can prevent formation of spent toner. However, abrasion, stripping, or cracks are not prevented satisfactorily. Furthermore, although an appropriate charge amount can be obtained for a positively charged toner, the charge amount is too small when a negatively charged toner is used, so that a large amount of oppositely charged toner (positively charged toner) is generated. Consequently, fog or toner scattering is aggravated, and thus the carrier may not withstand use. Furthermore, the transfer efficiency may be lowered.
In other words, a carrier having a resin coating layer containing only fluororesin can be used only for a positively charged toner due to the position in the triboelectric series, and the adhesive strength of the coating resin at an adhesive interface with the carrier is poor. Furthermore, the carrier cannot obtain sufficient shock-resistance due to the insufficient strength of the resin as a coating agent, and thus it may not withstand use for a long period of time.
When a fluororesin and another resin are mixed and used, a uniform resin coating layer is difficult to obtain, there is a broad distribution of charge amounts, and image defects such as fog or toner scattering are caused. Furthermore, fluororesins have the characteristic of moving to the outermost layer of the resin coating layers when mixed and used with another resin, and thus the charge amount decreases extremely for a negatively charged toner. In addition, when the coating layer is stripped by a long period of use, the fluororesin is stripped first, and the binder resin appears at the outer layer with the time of usage. Thus a change in, for example, charge characteristics becomes significant.
In recent years, it has been increasingly required to reproduce uniformly an image including a large amount of solid portion such as barcodes or an image such as graphic designs, instead of, for example, documents including a large amount of printed letter printed by, for example, printers. For example, particularly in full color development, solid portions are larger than text portions, and thus the amount of consumed or supplied toner increases, and it is desirable that the toner maintains the desired charge characteristics all the time under various environmental conditions. In these recent electrophotographic processes with large toner consumption and high replenishment development conditions, the above-described carrier having a silicone resin and a silane coupling agent such as an aminosilane coupling agent can impart some charge to a negatively charged toner, and has some durability over a long period of use. However, the carrier cannot impart charge sufficiently to a small sized toner or a high density toner for high definition for use in recent printers or full color developing devices dealing with a large amount of solid portion, and cannot increase the charge amount instantly with respect to toner supplied at the time of printing. Ultimately, a sufficient durability cannot be attained at present.
In a carrier having a resin coated-layer in which a terminal perfluoro alkylsilane coupling agent or a fluorine-substituted alkyl group is introduced to a silicone resin, although some improvement of the formation of spent toner can be confirmed, an appropriate charge amount cannot be obtained when used for a negatively charged toner. Furthermore, the coating film is not sufficiently uniform, and the carrier cannot satisfactorily prevent abrasion or stripping of the resin coating layer caused by downsizing of devices to cope with the recent space-saving trend and by increased stress on the carrier in a developing device in accordance with realization of high speed performance.